1. Field of the Invention
The present invention relates to a metal cord capable of increasing rubber permeability to an inner portion of a cord while making a diameter of cord compact, and a radial tire for a heavy load using the metal cord for a carcass.
2. Description of the Prior Art
In the carcass of the radial tire for the heavy load, a metal cord obtained by bundling a plurality of filaments is in heavy usage. This kind of metal cord is structured such that a plating treatment is applied to a surface thereof, however, for example, as shown in FIG. 10A, in the case of a compact cord a having no gap between filaments f and f, there is formed a space in which no rubber is permeated between the filaments. Accordingly, there has been a problem that a rust is generated within the cord due to an influence of a water content, thereby causing a reduction of bonding force between the cord and the rubber, a reduction of strength of the cord, a breakage of the cord and the like.
Then, in recent years, in order to improve the problem mentioned above, there has been proposed a so-called open cord b obtained by bundling so that a gap is generated between the filaments as shown in FIG. 10B, and a metal cord c obtained by bundling a filament f1 modeled in a spiral three-dimensional shape and a non-waved filament f2 so as to form a gap between the filaments, thereby increasing a permeability of the rubber, as shown in FIG. 10C.
However, in the cords b and c mentioned above, in order to secure a sufficient permeability of the respective rubbers, there is a disadvantage that a cord diameter is inherently increased.
Accordingly, an object of the present invention is to provide a metal cord which on the basis of using a metal cord of a layered structure forming a core by adding a torsion to one to four filament bundles having two-dimensional waved filaments and non-waved filaments in a mixed manner, can make a two-dimensional wave three-dimensional wave in accordance with the torsion and can sufficiently secure a rubber permeability while making a cord diameter compact, and a radial tire for a heavy load using the metal cord for a carcass.
Further, in accordance with the present invention, another object is to establish both of a cord strength reduction and a rubber permeability by limiting a wave height, a wave pitch and the like of the modeling of the waved filament to predetermined ranges, and to intend a stability of a tire shape by restricting an extension of the cord at a time of a low load.
In order to achieve the object mentioned above, in accordance a first aspect of the present invention, there is provided a radial tire for a heavy load including:
a carcass extending to a bead core of a bead portion from a tread portion via a side wall portion; and
a belt layer arranged inside the tread portion and outside the carcass,
wherein a carcass cord of said carcass is constituted by a metal cord having a core comprising totally 6 to 12 filaments sectioned into 1 to 4 filament bundles, and a sheath comprising 8 to 15 filaments arranged around the core,
wherein each of said filament bundles includes a waved filament modeled in a two-dimensional wave shape having crests and troughs repeatedly and a non-waved filament, in a state before being bundled, and
wherein said core makes the wave of said waved filament three-dimensional within the core by applying a torsion to each of the filament bundles.